1. Field of the Invention
The present invention relates to a wide-band receiver which receives intended system bands and selects a channel by means of digital processing.
2. Related Art Statement
Research and development on various radio communication systems have been conducted in the field of data transmission and the like lately to deal with the increase of an amount of information to be transmitted and to accommodate to characteristics of each radio communication system. Such plurality of radio communication systems transmit a plurality of channels mixedly.
FIG. 1A is a circuit diagram showing the related art of a receiver which receives and demodulates data of one communication system among such various radio communication systems. This device has been disclosed in First Document (P. R. Gray and R. G. Mayer, “Future directions of Silicon IC's for RF personal communications,” Proc. Custom Integrated Circuits Conference '95, pp. 83–90, 1995).
The signal received by the device of FIG. 1A is what data is multiplexed and transmitted by modulating into a high-frequency signal band by orthogonal axes (I, Q axes). A high-frequency signal (RF signal) of such high frequency fRF is inducted at an antenna 1. The band of the RF signal is limited by a band-pass filter 2 which is an RF filter and is supplied to analog mixers 3 and 4 which compose an orthogonal demodulating section.
A local oscillator 5 outputs a local oscillating output whose oscillating frequency fC is fixed to a phase shifter 6 to convert the RF signal into a base-band signal. The phase shifter 6 shifts the phase of the local oscillating output by π/2 and supplies the local oscillating outputs which are orthogonal to each other to the mixers 3 and 4. The mixers 3 and 4 convert the frequency to that of the base-band by multiplying the inputted RF signal with the local oscillating outputs.
The outputs of the mixers 3 and 4 are limited by analog low-pass filters 7 and 8 and are supplied to analog multipliers 9 through 12 which compose an orthogonal demodulating section. A local oscillating output from a local oscillator 13 is phase-shifted by π/2 by a phase shifter 14 and is inputted to the multipliers 9 through 12. The local oscillating frequency fCK of the local oscillator 13 is variable. That is, the local oscillating output from the local oscillator 13 is set at frequency for demodulating a communication system (channel) to be selected.
The multipliers 9 and 11 multiply the local oscillating output which is orthogonal each other to the output of the low-pass filter 7 and the multipliers 10 and 12 multiply the local oscillating output which is orthogonal each other to the output of the low-pass filter 8. An I axis signal may be obtained by adding the outputs of the multipliers 9 and 10 by an analog adder 15 and a Q axis signal may be obtained by adding the outputs of the multipliers 11 and 12 by an analog adder 16. Image components may be rejected by the adding process of the adders 15 and 16.
Analog low-pass filters 17 and 18 limit the band of the outputs of the adders 15 and 16 and supply them to A/D converters 19 and 20. The A/D converters 19 and 20 convert the analog signals of the I and Q axes into digital I and Q signals and output them.
FIG. 1B is a block diagram representing the circuit diagram of FIG. 1A.
That is, as it is apparent by comparing FIG. 1A with FIG. 1B, an RF filter 21 corresponds to the band-pass filter 2, an orthogonal demodulating section 22 corresponds to the mixers 3 and 4 and filters 7 and 8, a fixed local oscillating section 23 corresponds to the local oscillator 5 and the phase shifter 6, an orthogonal demodulating section 24 corresponds to the multipliers 9 through 12, a variable local oscillating section 25 corresponds to the local oscillator 13 and the phase shifter 14, an image rejecting section 26 corresponds to the adders 15 and 16, a channel selecting section 27 corresponds to the filters 17 and 18 and an A/D converter section 28 corresponds to the A/D converters 19 and 20.
That is, the device shown in FIG. 1B is arranged so as to select a desirable wave by the analog channel selecting section 27 after rejecting the image component of the output of the orthogonal demodulating section 24 by the image rejecting section 26 and to output by converting it into digital signals by the A/D converter section 28.
However, the image rejecting precision is low due to the incompleteness of the analog circuit of the mixers 3 and 4, the filters 7 and 8, the phase shifter 6 and the analog multipliers 9, 10, 11 and 12. Further, such arrangement lacks flexibility in changing bands and cutoff in correspondence to a plurality of systems because the filters composing the channel selecting section 27 are analog filters 17 and 18.
FIG. 2A is a circuit diagram showing another exemplary related art of such receiver. This device has been disclosed in Second Document (J. Crols and M. Steyaert, “A single-chip 900 MHz CMOS receiver front-end with a high performance low-IF topology” IEEE J. Solid-State Circuits, vol. 30, No. 12, pp. 1483–1492, 1995).
FIG. 2B is a block diagram representing the circuit diagram of FIG. 2A. The device in FIGS. 2A and 2B is characterized in that A/D converters 31 and. 32 which correspond to an A/D converter section 46 are provided before multipliers 33 through 36 which correspond to an orthogonal demodulating section 47. The oscillating frequency fC of a local oscillator 30 is variable. The local oscillator 30 composing a variable local oscillating section 45 outputs an oscillating output of oscillating frequency corresponding to a channel to be selected. This oscillating frequency sets the output of the mixers 3 and 4 at frequency corresponding to fixed oscillating frequency fCK of a local oscillator 37 composing a fixed local oscillating section 48.
The outputs of the low-pass filters 7 and 8 are converted into digital signals by the A/D converters 31 and 32 and are then supplied to the digital multipliers 33 through 36 composing an orthogonal demodulating section 47. The oscillating output of the local oscillator 37 whose phase has been shifted by π/2 by a phase shifter 38 is given to the digital multipliers 33 through 36. Signals of I and Q axes may be obtained by the multipliers 33 through 36. The output of the multipliers 33 and 34 and the output of the multipliers 35 and 36 are subtracted respectively by adders 39 and 40 composing an image rejecting section 49 and rejected image components are supplied to low-pass filters 41 and 42. The digital low-pass filters 41 and 42 composing a channel selecting section 50 output by limiting the band of the signals of I and Q axes.
The frequency of the variable local oscillating section 45 (local oscillator 30) is changed in selecting a desirable channel in this arrangement. That is, it is provided with the function of selecting a channel by the orthogonal demodulating section 22 and the variable local oscillating section 45. However, because the variable local oscillating section 45 is an analog high-frequency oscillator, it has been difficult to change the frequency programmably and to vary the frequency per channel interval across a wide-band. Further, because the oscillating frequency of the local oscillator 30 exists within the band of the desirable channel in this arrangement, it has been unable to provide the RF filter 21, differing from the case of FIGS. 1A and 1B. Accordingly, the image rejecting degree which has been realized by the RF filter 21 cannot be obtained, thus degrading the image rejecting degree of the whole receiver system.
FIG. 3A is a circuit diagram showing another exemplary related art of the receiver. This device has been disclosed in Third Document (J. Crols and M. Steyaert, “Low-IF topologies for high-performance analog front ends of fully integrated receivers,” IEEE Trans. Circuits & Syst., vol. 45, No. 3, pp. 269–282).
FIG. 3B is a block diagram representing the circuit diagram of FIG. 3A. The device shown in FIG. 3A is what the low-pass filters 41 and 42 in the device of FIG. 2A are eliminated and band-pass filters 51 and 52 are adopted instead of the low-pass filters 7 and 8, respectively. The band-pass filters 51 and 52 select a channel together with the mixers 3 and 4 and the local oscillator 30. That is, the device in FIG. 3A selects a desirable channel by the analog band-pass filters 51 and 52 after setting the frequency of the local oscillator 30 at the desirable channel and demodulating orthogonally similarly to the device in FIG. 2A.
This arrangement disallows the image rejecting degree to be obtained by the RF filter similarly to the device in FIG. 2B and the band width of the band-pass filters 51 and 52 must be varied in analog when channel bands are different.
Thus, the receivers of the related art shown in FIGS. 1A through 3A have realized the selection of a channel by means of the analog circuits. By the way, it has come to be required to be able to receive signals of a plurality of systems by one terminal lately. It is desirable to select a channel by means of digital processing to flexibly accommodate with each system because a band width per channel and characteristics of a waveform shaping filter are different among different systems.
Due to that, there has been required a wide-band receiver which receives the whole bands of radio communication systems to be received in batch. A direct conversion receiver has been drawing an attention lately as a radio scheme for realizing the wide-band receiver.
FIG. 4 is a circuit diagram showing the structure of a related art of the receiver considered to be such direct conversion scheme receiver.
A high-frequency signal received by an antenna 1 is multiplied with a carrier wave of fixed oscillating output supplied from a local oscillator 5 via a phase shifter 6 by mixers 3 and 4 distributed into two systems and is frequency-converted into a base-band frequency band in batch per system band containing a desirable wave to be received. The oscillating frequency of the local oscillator 5 of fixed oscillating output is set within the frequency band of the system to be received.
The output of the local oscillator 5 is supplied to the mixers 3 and 4, respectively, via the phase shifter 6. Accordingly, signals of two systems of I and Q frequency-converted into the base-band have a phase difference of π/2 from each other. The band of the base-band signals of the two systems are limited by anti-aliasing low-pass filters 7 and 8. Then, a π/2 phase shifter 61 shifts the phase of only one channel by π/2 and an adder 62 subtracts them to reject an image component.
An A/D converter 63 converts the output of the adder 62 in batch per system band. Then, a digital processing section 64 comprising digital multipliers 65 and 66, a digital π/2 phase shifter 68, a local oscillator 67 of variable oscillating output and digital filters 69 and 70 carries out digital orthogonal demodulating operations. The digital filters 69 and 70 digitally select a desirable signal from the orthogonal demodulated outputs from the multipliers 65 and 66, thus obtaining I and Q signals.
Next, the above-mentioned operations will be explained on a frequency axis by using FIGS. 5A through 5C.
Now assume a case of receiving eight channels in batch in a radio communication system including signals of the eight channels of channel 1 (ch1) through channel 8 (ch8) and of selecting only hatched ch6 as a desirable wave. When the oscillating frequency of the local oscillator 5 is set at the frequency between ch4 and ch5 as shown in FIG. 5A, ch3 turns out to be an image signal (grid pattern) for ch6.
The output of the adder 62 turns out as shown in FIG. 5B by the orthogonal demodulation carried out by the multipliers 3 and 4. That is, the receiver receives the system bands in batch. In this case, the image signal ch3 is rejected. Next, the receiver A/D-converts the signal shown in FIG. 5B and then orthogonally demodulates by the digital processing section 64. In this case, the oscillating frequency of the variable local oscillator 67 is tuned to the desirable channel. That is, it is tuned to the frequency of ch6. Thus, the orthogonal demodulated output shown in FIG. 5C is obtained from the digital processing section 64. The desirable channel ch6 becomes a DC (direct-current) component to be selected by the digital filters 69 and 70. Thus, the device in FIG. 4 allows the desirable channel to be selected digitally while receiving the system bands in batch.
However, actually, the analog orthogonal demodulating section does not operate ideally due to the unbalance of the phase of amplitude between the I and Q channels of the mixers 3 and 4 and the low-pass filters 7 and 8 composing the analog orthogonal demodulating section. Therefore, there has been a problem that the rejection of image component is not carried out fully.
FIGS. 6A through 6C are frequency spectrum diagrams for explaining this problem. FIGS. 6A through 6C correspond to FIGS. 5A through 5C, respectively. Because the analog orthogonal demodulating section does not operate ideally, the image signal (grid pattern) of ch3 remains in the output of the adder 62 while superimposing to the desirable channel ch6 (hatched channel) as shown in FIG. 6B. The ratio of the component of ch6 to the image signal component of ch3 is the image rejecting degree as shown in FIG. 6B. After all, the image signal component remains in the orthogonal demodulated output of the desirable channel (FIG. 6C).
In case of a circuit of 2 GHz band for example, only image rejecting degree of around 30 dB can be realized in the adder 62 which corresponds to the image rejecting section. A value of 60 to 70 dB is required as the image rejecting degree in normal radio communication systems. Therefore, it is unable to obtain the image rejecting characteristics fully depending on a system by the arrangement of FIG. 4.
Thus, the related arts of the receivers described above have had problems that the precision of the orthogonal demodulator is relatively low due to the incompleteness of the analog section and that it is unable to obtain the image rejecting characteristics fully when the wide-band receiver which receives the bands of systems to be received in batch and selects a channel by means of digital processing is to be realized.